Aromatic polyhalogenated halomethyl compounds are known as important building blocks for a variety of chemical products. These intermediate compounds have the advantage of good chemical reactivity and extremely high halogen content. Their major applications are in the production of flame retardants. Several processes for production of these intermediates are known in the literature.
For example a one pot synthesis using carbon tetrachloride as solvent and benzoyl peroxide as radical source was described by Dong S. et al. in Jingxi Huagong 14(3) (1997) 35-36. The yield of this process was 86% and the choice of the highly toxic solvent and the initiator are not easily applied on an industrial scale.
Another process was described by Shishkin, S. et. al. in Zhurnal Organicheskoi Khimii 17(6) (1981) 1270-5. Aromatic bromination of toluene was achieved using iron as catalyst, and the side chain bromination was done using either bromine with n-bromosuccinimide and benzoyl peroxide in carbon tetrachloride or with bromine in carbon tetrachloride using UV-irradiation at 80% to 86% yields.
DE 3,828,059 describes a process for side chain halogenation using a solvent and a catalyst such as nickel, cobalt, platinum or their salts.
U.S. Pat. No. 4,212,996 discloses a process for side chain chlorination of aromatic compounds completely halogenated in the nucleus and containing methyl groups. The process is a chlorination of side chains but the preparation of pure pentabromobenzyl bromide (PBB-Br) is also described. This was done in hexachlorobutadiene, HCBD, at 175° C.-185° C., using pentabromotoluene and bromine in the presence of 2,2′-azobis(isobutyronitrile).
In U.S. Pat. No. 6,028,156 a process for preparation of PBB-Br is described (Example 5). This process uses pentabromotoluene in chlorobenzene with bromine and 2,2′-azobis(isobutyronitrile) (AIBN). The process is actually a one-pot process for preparation of pentabromobenzyl acrylate (PBBMA) from pentabromotoluene (5BT) without isolating the PBB-Br.
Whether the final product is a flame retardant by itself or an intermediate for the production of the final flame retardant compound, the purity of these aromatic polyhalogenated halomethyl derivatives is a key issue.
In general the production of aromatic polyhalogenated halomethyl compounds involves two chemical stages: polyhalogenation of a methyl-aromatic compound and halogenation of the methyl group. The order of these two distinct chemical steps is one of the differences between known processes. If for example toluene is chosen as raw material, polybromination of the methyl-aromatic skeleton would result in pentabromotoluene. The pentabromotoluene is then chlorinated or brominated in the second chemical stage to produce pentabromobenzyl chloride or pentabromobenzyl bromide. On the other hand, toluene could be used to produce first the benzyl bromide or chloride necessary for the second stage of perbromination. Most known processes prefer to start with toluene and proceed via the pentahalotoluene to the pentahalobenzyl derivative.
The first chemical stage of aromatic perbromination is usually performed with the aid of a Lewis acid catalyst in a suitable, dry solvent. Such solvents are commonly dihalomethanes or their mixtures, dihaloethanes or their mixtures, and other solvents inert to bromine or chlorine in the presence of Lewis acid catalysts. Bromine itself can also be used as both the reagent and the solvent in such a process. The appropriate choice of the Lewis acid catalyst is also of high importance when the perbrominated product is intended to undergo another step of side chain bromination. Trace amounts of residual catalyst can strongly influence the side chain bromination as well as the color of the final product.
The second chemical stage, selective mono-halogenation of the methyl group, also known as benzylic halogenation, is achieved by a radical process, using some source of radical initiator to convert the bromine or chlorine molecule into reactive radicals that attack the methyl group to form the halomethyl functionality. The choice of radical source is rather limited for an industrial process, while the influence of this type of initiator on the final purity of the product is significant.
One of the most suitable radical initiators for this purpose is AIBN, 2,2′-azobisisobutyronitrile, CAS RN [78-67-1], 2,2′-azobis(2-methyl-proprionitrile).
The decomposition of AIBN is essential for the benzylic halogenation, side chain bromination, to proceed since the radicals formed by decomposition of AIBN initiate the formation of bromine radicals which are the active brominating species in this type of process. When elevated temperatures are applied, as described in prior art, the decomposition of AIBN is too fast, the major part of the radicals formed is consumed in side reactions of the methyl group or of the solvent. Therefore the inventors have found that it is highly recommended to perform such reactions at temperatures that will ensure high selectivity together with reasonable reaction time. At too low temperature the formation of radicals will be slowed down so that no effective reaction will occur.
The inventors have also found that the presence of an appropriate amount of water is essential for high efficiency of benzylic bromination. An equivalent amount of HBr is formed and must be efficiently removed in order to minimize the formation of Br3. species which cannot give Br radicals upon encounter with AIBN radicals.
When two chemical stages are involved in the production of a new compound, it is always desirable, from the economic point of view, to perform both stages in the same pot without isolating the intermediate. On the other hand, it is difficult, in most cases, to obtain a product of high purity if the intermediate is not isolated and purified, since byproducts of the first stage also become involved in the chemistry of the second stage, increasing the range of impurities formed in the process. The process of the invention makes it possible to obtain a product of high purity even when the intermediate was not isolated and the process was performed in a one-pot manner.
The application of aromatic polyhalogenated halomethyl compounds as flame retardants or as intermediates for the production of other flame retardant compounds such as pentabromobenzyl acrylate (PBBMA) and polypentabromobenzyl acrylate (PBBPA) dictates the maximum allowed level of byproducts so that an optimal performance is achieved. For example, JP 11279381 (Application No. JP 98-85341) emphasizes the advantages of high purity polypentabromobenzyl acrylate containing no more than 1500 ppm of residual pentabromotoluene.
As will be appreciated by the skilled person, any flame retardant that is of higher purity, higher thermal stability and lower coloration, achieved by lower levels of foreign materials, will be more successful as a flame retardant than the same active ingredient with inferior properties.
PBB-Br has different uses, some of which may have less stringent quality requirements. However, the applicant hereof has found that in order to use PBB-Br as a flame retardant in polystyrene very specific conditions must be met in order to obtain high-quality products. The use of PBB-Br in foamed polystyrene is the subject of a separate patent application of the same applicant hereof (IL 163100, filed on Jul. 19, 2004).
The most important properties that should be fulfilled by a brominated organic compound, specially designed for flame retarding polystyrene articles, are:                1. Good uniform dispersion of the brominated additive in the polystyrene matrix. Such uniform dispersion is best achieved when the melting range of the brominated additive is lower than the typical processing temperature of the polystyrene, so that during processing the flame retardant additive is uniformly distributed throughout the polystyrene resin.        2. High thermal stability of the brominated organic flame retardant is another crucial property since additives of low thermal stability will limit the possibilities of regrinding and recycling the flame retarded material. Flame retardant additives of insufficient thermal stability will cause degradation of the polystyrene resin, by reducing the molecular weight of the styrene polymer foam and this in turn will immediately cause a drop in all mechanical and insulating properties of the foam, and even corrosion of the equipment in the most severe cases.        
In principle there are several ways to ensure the necessary thermal stability of a chosen flame retardant molecule; among those the chemical purity of the compound directly influences its thermal stability.
It is a purpose of the present invention to provide an improved process that results in a product of high purity, suitable for use as an intermediate for the production of flame retardant agent or for direct application in polymeric resins as a flame retardant.
It is another purpose of the present invention to provide a process for production of aromatic polyhalogenated halomethyl compounds, especially pentabromobenzyl bromide, PBB-Br, of high quality as defined above, so that optimum performance is achieved when applied directly as a flame retardant in polymeric resins such as polystyrene or when used for the production of other flame retardant compounds such as PBBMA.
It is a further purpose of the present invention to provide a process for the production of PBB-Br that can be carried out either as a one-pot process or in two distinct chemical steps.